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On the presence of hydrous defects in differently coloured wulfenites (PbMoO4): an infrared and optical spectroscopic study

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TALLA Dominik WILDNER Manfred BERAN Anton ŠKODA Radek LOSOS Zdeněk

Year of publication 2013
Type Article in Periodical
Magazine / Source Physics and Chemistry of Minerals
MU Faculty or unit

Faculty of Science

Field Geology and mineralogy
Keywords wulfenite;OH defects
Description Several samples of wulfenite, PbMoO4, varying in colour from colourlerss to yellow, orange and red, have been analyzed by means of IR and optical absorption spectroscopy and by microprobe analyses. A distinct pleochroic band group with absorption maxima centred at 3,380 and 3,150 cm-1 can be seen in the IR spectra of wulfenite single-crystals, indicating the presence of hydroxyl groups. The pleochroic and thermal behaviour of the OH stretching bands along with deuteration experiments, as well as results obtained from synthetic fluxgrown samples, exclude the presence of submicroscopic hydrous mineral inclusions as their primary origin. The pleochroic scheme and band positions were used to postulate a model for the OH incorporation mode, based on the assumption of vacancies on Mo and Pb sites in the structure of this ‘nominally anhydrous’ mineral. Optical absorption spectra of coloured natural samples show a broad and polarised band around 23,000-24,000 cm-1, preceding the fundamental UV absorption edge, which has been identified as the reason for the colour of the mineral. The comparison with synthetic PbMoO4, single-crystals, doped with variable amounts of Cr6+, yielded conclusive evidence that trace amounts of the CrO4 2- anion group, substituting for MoO4 2-, determine the variable colour. Besides, in one sample, trace amounts of Nd3+ have been spectroscopically identified.
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